Light-Emitting Diodes (“LEDs”) are semiconductor light sources. With the development of high efficiency and high power LEDs, LEDs have been widely used for lighting and illumination sources. While monochrome displays can use colored light sources, such as electroluminescent back lights or colored LEDs, color displays still require a white LED as a light source to properly display color.
Using LEDs for illuminations has a wide rage of applications, such as backlighting for Liquid Crystal Display (LCD) in handheld devices (e.g. cell phone, MP3, MP4, GPS, PDAs, digital cameras, etc.), backlighting for notebook computer displays, backlighting for LCD televisions, and the like.
There are two main methods for providing a white light source: white LEDs and Cold Cathode Fluorescent Lamps (“CCFLs”). CCFLs have been used for years in notebook computer display and Televisions. However, comparing the CCFL, the advantages of using LED array as the LCD backlight source lies in, for example, high brightness, high contract ratio, fast reaction rate, wide color range, lower power consumption, long life-span, and reduced environmental pollution, to name but a few. Therefore, LEDs are becoming the preferred light source for in a variety of applications from consumer devices to industrial lighting.
There are several methods of backlighting an LCD panel using multiple LEDs or LED arrays, such as positioning white LED arrays behind the LCD panel or using Edged-LED lighting. Edged-LED lighting uses multiple white LEDs arranged around the inside frame of the display along with a special light diffusion panel designed to spread the light evenly behind the LCD panel.
LED light characteristics are generally described with a function of LED working current. To control the brightness of an LED is to control the working current of the LED. LED circuits can be driven in series or in parallel and they both have their own advantages and disadvantages.
In series connection, multiple LEDs can be connected in series with a single current limiting resistor provided the source voltage is greater than the sum of the individual LED threshold voltages. The disadvantage of series connection is that it requires a higher supply voltage and is not power and energy efficient.
In parallel connection, multiple LEDs can be connected in parallel, but the LEDs must have closely matched forward voltages in order to have equal branch currents and, therefore, generally equal brightness. Variations in the manufacturing process can make it difficult to obtain exactly equal forward voltages and, therefore, equal current or equal brightness when connecting some types of LEDs in parallel.
Therefore, it would be desirable to have a system and method for creating an LED-based lighting system that is efficient and provides a consistent and substantially uniform light and brightness.